Electrolyte



Patented Apr. 23, 1940 UNITED STATES ELECTROLYTE Alexander M. Georglev,Dayton, Ohio, asslgnor to General Motors Corporation, Detroit, Mich., acorporation oi Delaware No Drawing. Application January 22, 1937, SerialNo. 121,823

, 14 Claims.

This invention relates to electrolytes for electrolytic cells such aselectrolytic condensers, rectifiers and the like; and. while not at alllimited thereto, the electrolyte of this invention is particularlyadapted for use in aluminum electrode condensers which have formed onthe electrodes a dielectric film, and especially those of the socalleddry type in which the electrolyte is substantially absorbed in anabsorbent material, the

latter serving also as a spacer between the electrodes.

The application is a continuation in part of my pending application35,263, filed August 8,

In general there are certain qualities that an electrolyte shouldpossess, to be entirely satisfactory in an electrolytic condenser. Thatis:

(1) The electrolyte should not attack the electrodes, the film on theelectrodes, the terminals,

connections or container, or the spacer of the cell.

(2) The electrolyte must be an effective filmforming andfilm-maintaining agent.

(3) The electrolyte should be substantially 5 permanent in its chemicalconstitution and physical properties. Hence, it should not be apt todecompose under operating conditions, or exhibit precipitation andstratification or undergo other changes.

(4) The electrical resistance of the electrolyte should not be too low,because if it is, the protective film is weakened and the breakdownvoltage is lowered. However, on the other hand, the resistance shouldnot be too high, because the electrical losses are thereby increased andthe efiiciency of the cell is reduced.

(5) The electrolyte should readily penetrate into the pores and/orinterstices of the spacer when used in dry type cells. This facilitatesimpregnation and provides for thorough covering of the electrodes.

(6) In some instances, the variations of the electrical characteristicsof the cell, such as capacity, power factor, leakage current andbreakdown voltage, with temperature changes are important. Thesevariations of the characteristics of the cell depend very materiallyupon the properties of the electrolyte used. Hence, the electrolyte ispreferably chosen to meet the specific operating requirements of thecell.

Electrolytes, such as those containing glycerol and a boric acidcompound have been known in the art. Hence, to provide a betterunderstanding u of the present invention, some of the distinctions andadvantages of this electrolyte over those previously known will bediscussed.-

The usual electrolytes prepared from glycerol and boric acid compoundsexhibit marked acidity and are frequently apt to attack the electrodes;5 while electrolytes prepared in accordance with the present inventionare almost neutral within a wide range of proportions of theingredients, and do not attack the electrodes. Furthermore, electrolytes prepared in accordance with this inven- 10 tion do notnoticeably attack the film, even during long periods in which the cellis not in use. Rather, the electrolyte has good film-forming andmaintaining properties, indicated by low leakage current. 15

Ammonium acetate, or acetic acid and ammonium hydroxide are more solublein a polyhydroxy alcohol, such as glycerol or ethylene glycol, than isboric acid. The electrolyte solution is therefore easier to make. 20

Being readily soluble in glycerol or ethylene glycol, the ionogen ofthis invention permits wide variations in cell characteristics, so thatcells can be easily adapted to widely varying services, such as use onhigh or low voltage, where high or low 5 capacity is desired, use inintermittent or continuous service, use on pure alternating current orwhere direct and alternating currents are superimposed, and use underdifierent operating temperature conditions. 30

An electrolyte utilizing ammonium acetate or acetic acid and ammoniumhydroxide admixed with glycerol has a lesser tendency toward producing aprecipitate than do some of the other electrolytes. This factor is ofconsiderable im- 3 portance, because with the present composition, theelectrolyte is more uniform throughout the cell in which it is used.

It has also been found that some electrolytes, such as some of thoseutilizing glycerol and a 40 boric acid compound, assume a gummy orviscous character, or harden when subjected to heat that occurs undersevereoperating conditions. An electrolyte prepared in accordance withthis invention, with ammonium acetate, or its equiva- 4,5 lent, as theionogen does not exhibit such tendency to harden, even when heated toquite severe and unusual temperatures.

Furthermore, with a small proportion by weight of ammonium acetate oracetic acid and ammonium hydroxide in the electrolyte, a greaterconductivity of the electrolyte results than when other materials, suchas boric acid compounds, are used. Consequently, by utilizing thepresent invention, an electrolyte having conductivity that issufliciently high for proper and very satisfactory use can be preparedthat is very fluid, and will easily penetrate the absorbent materialused in the dry type of electrolytic condensers, even when they areimpregnated in rolled or assembled form. The latter advantage inimpregnation is important in the dry type of electrolytic condensers,because thorough impregnation of the condenser is essential, and becauseit makes such thorough impregnation possible in a shorter time and at alower temperature. The lower temperature of impregnation makes theprocess simpler and minimizes the danger of losing some of thecomponents of the electrolyte due to evaporation.

It is therefore an object of this invention to provide an electrolytefor electrolytic cells that utilizes materials that are readily solublein substances, such as a polyhydroxy alcohol.

It is another object of this invention to provide an electrolyte for theelectrolytic cells that minimizes the tendency to form any precipitatetherein, and therefore is and remains substantially uniform throughoutthe cell in which it is used.

Another object of this invention is to provide an electrolyte forelectrolytic cells which has a sufficiently high value of conductivitywhen only a relatively small amount of material is added to the solvent,so that the electrolyte is relatively thin in consistency at normaltemperatures.

Another object is to provide an electrolyte for electrolytic cells thatis easy and economical to manufacture and to introduce into the cells inwhich it is used.

Another object of this invention is to provide an electrolyte forelectrolytic cells that is substantially stable physically andchemically under operating conditions over a wide range of ionogenconcentrations.

Another object of this invention is to provide an electrolyte forelectrolytic cells that has good film-forming and maintaining qualities,and does not attack the film, electrodes, terminals or any parts incontact therewith.

Another object of this invention is to provide an electrolyte forelectrolytic cells that is relatively fluid at normal temperatures, evenwhen relatively high percentages of the ionogen are used, so thatheating to high temperatures during impregnation of absorbent spacertype units is unnecessary.

Another object of this invention is to provide an electrolyte forelectrolytic cells that does not get gummy or hardened under severeoperating conditions.

It is also an object to provide a method by which an electrolyte havingone or more of the above qualities can be produced.

Further objects and advantages of the present invention will be apparentfrom the following description.

As previously mentioned, the electrolyte of the present invention ismore particularly adapted to use in the electrolytic cell itself, andpreferably in cells having electrodes upon which the film has beenformed. The cell may be of either the wet or dry type, but theelectrolyte of this invention is very satisfactory for cells of the drytype having an absorbent material for absorbing the electrolyte andspacing the electrodes. Therefore, although not limited thereto, thedescription will include the particular steps necessary in adapting theelectrolyte to the dry type' of cell.

Broadly, the electrolyte of the present inven tion comprehends apolyhydroxy alcohol, such as ethylene glycol or preferably substantiallydry glycerol, having added thereto either acetic acid and ammoniumhydroxide or ammonium acetate. It is understood that there is a chemicalreaction between acetic acid and glycerol when acetic acid is added tothe glycerol. It is also understood that further chemical reaction takesplace when ammonium hydroxide is added to the substance resulting fromthe mixture of glycerol and acetic acid. Since electrolytes havingsimilarly good properties can be obtained by making an electrolyte byeither separately adding acetic acid and ammonium hydroxide or by addingammonium acetate to a polyhydroxy alcohol, such as glycerol or ethyleneglycol, it is not deemed particularly vital to this discussion whetherthe final chemical make up of the electrolytes thus made is exactly thesame or not. The control of the acidity of the electrolyte is easierwhen the acetic acid, polyhydroxy alcohol and ammonium hydroxide areused. Such acidity control may be desirable to adapt a condenser to aparticular use. However, both electrolytes are so. similar that theyboth are clearly within the scope and spirit of the present invention.It is further understood that instead of ammonium hydroxide, ammonia gascan be used to effect substantially the same result as that obtained bythe use of ammonium hydroxide, such as by bubbling the gas through thesubstance as a step in the process of manufacture.

Although not desiring to be limited in any way to the particularproportions or percentages of substances, the following is an example ofan electrolyte and a process for making the same that comes within thescope of the present invention, and contains proportions of ingredientsthat are near the preferable minimum limit of the ionogen concentration.A polyhydroxy alcohol, preferably substantially dry and pure glycerol,is thoroughly admixed with acetic acid, preferably glacial acetic acid,in proportions of 98 percent glycerol to two percent glacial aceticacid. Then, to this is added ammonium hydroxide, preferably 26 percentsolution of ammonia water, to the extent of five percent of the totalweight of glycerol and acetic acid.

An electrolyte made according to the above described exemplaryproportions, although quite flowable, is preferably heated in anautoclave or vessel adapted to be substantially sealed with respect tothe atmosphere or opened. The heating further facilitates impregnationof the con denser units: while the heating and autoclave provide forcontrol of evaporation and moisture content of the electrolyte. Byopening the auto clave when heat is applied to the electrolyte, themoisture content of the electrolyte is reduced; while closing of theautoclave during impregnation prevents appreciable change of moisturecontent or evaporation of electrolyte during impregnation, although heatis applied to the electrolyte to facilitate impregnation. The heatingreduces the viscosity of the electrolyte and increases the rate ofpenetration of the electrolyte into the absorbent material. It isgenerally only desirable to drive off moisture in the control of thewater or moisture content of the electrolyte when it is made by usingammonium hydroxide as described above.

To illustrate and emphasize the fact that although the proportions ofthe electrolyte and the method of preparation given above may bepreferred for condensers or the like having particular characteristics,the proportions of the polyhydroxy alcohol, acetic acid and ammoniumhydroxide or polyhydroxy alcohol and ammonium acetate may be widelydiiferent, and such different proportions might well be preferred ior acondenser or cell where different characteristics are desirable. As afurther example and again not as a limitation upon the invention, it ispointed out that an electrolyte having ashigh as 40 percent by weight ofammonium acetate and substantially 60 percent of polyhydroxy alcoholdoes not tend to form a precipitate over long periods of time, is stablein electrolytic condenser cells and is very fiowable to an extent thateven condensers of the rolled type having dense paper, such as linenpaper separators, can be easily and thoroughly impregnated. Such densepaper is particularly desirable in condensers for use on highervoltages, because it insures a higher safety factor. At present moreabsorbent paper is commonly used, although it does not insure a veryhigh safety factor but because it will absorb the highly viscouselectrolytes that are now used in the art. Furthermore, other thingsbeing equal, the more flowable electrolyte is also more desirable,because a greater quantity will be absorbed, thus insuring longer lifeof the condenser, particularly under adverse conditions.

As an example of the advantage of having available such a wide range ofvariation of the proportions of the electrolyte ingredients, it ispointed out that for best results certain proportions andcharacteristics are preferable, depending upon the operating conditionsfor which the cell is designed. That is, as a rule, condensers or cellsintended for use on higher voltages should have an electrolyte having ahigher electrical resistance, so that higher breakdown voltage isinsured. On the other hand, lower voltage condensers or cells canutilize an electrolyte having a lower resistance, which lower resistanceis preferable to reduce the electrical losses of the cell. In selectingthe electrolyte proportions, it is understood that within a wide range,increasing the percentage or concentration of the ionogen increases theconductivity of the electrolyte.

As previously emphasized, it is preferable to keep the concentrationbelow the point at which a precipitate forms which, for example, in thepresent instance is about 50 percent by weight of ammonium acetate. Onthe other hand, a concentration of ionogen that is too low as, forexample, one or two percent by weight of ammonium acetate-does not aseffectively maintain the dielectric film.

The above examples therefore give the preferred or desirable limitingfactors. It may be added, however, that for a dry electrolyticcondenser, such as those used for starting small alternating currentmotors of about 110 volts rating, particularly desirable results can beobtained when an electrolyte is used that has about 10 percent by weightof ammonium acetate and 90 percent of glycerol or ethylene glycol. Forcondensers such as this a preferable range of resistance for the cell isfrom 7,000 to 23,000 ohms per centimeter cube at 85 F. Satisfactorycondensers can be made for certain purposes having an allowable range ofresistance for the electrolyte of from 1,500 to 30,000 ohms percentimeter cube at 85 F.

While the embodiment of the present invention as herein disclosed,constitutes a preferred form, it is to be understood that other formsmight be adopted, all coming within the scope of the claims whichfollow.

What is claimed is as follows:

1. As a substantially neutral non-aqueous electrolyte of relatively highfluidity for an electrolytic cell, the combination including asingredients a polyhydroxy alcohol, acetic acid and ammonium hydroxide.

2. As a substantially neutral non-aqueous electrolyte of relatively highfluidity for an electrolytic cell, a composition comprehending thecombination of a polyhydroxy alcohol, glacial acetic acid and ammoniumhydroxide.

3. As a substantially neutral non-aqueous electrolyte of relatively highfluidity for an electrolytic cell, a composition comprehending thecombination of glycerol, acetic acid and ammonium hydroxide. I

i. As a substantially neutral non-aqueous electrolyte of relatively highfluidity for an electrolytic cell, a composition comprehending thecombination of a polyhydroxy alcohol and ammonium acetate.

5. As a substantially neutral non aqueous electrolyte of relatively highfluidity for an electrolytic cell, the combination comprehending thecombination of glycerol and ammonium acetate.

6. The methodof preparing an electrolyte for electrolytic cellscomprising the steps of admixing substantially dry glycerol and aceticacid in proportions of more than 40 percent glycerol and more than onepercent glacial acetic acid and then admixing therewith more than onepercent of the total weight of ammonium hydroxide.

7. The method of preparing an electrolyte for electrolytic cellscomprising the steps of admixing substantially dry glycerol and aceticacid in proportions of more than 40 per cent glycerol and more than onepercent glacial acetic acid and then admixing therewith more than onepercent of the total weight of a 26 percent solution of ammonia water.

8. The method of preparing an electrolyte for electrolytic cellscomprising the steps of admixing a polyhydroxy alcohol and ammoniumacetate in proportions of approximately 90 percent of the polyhydroxyalcohol and approximately 10 percent ammonium acetate.

9. As a substantially neutral non-aqueous electrolyte of relatively highfluidity for an electrolytic cell, the composition including thereaction products of the combination of a polyhydroxy alcohol, aceticacid and ammonia gas.

10. The method of preparing an electrolyte for electrolytic cellscomprising the steps of admixing a polyhydroxy alcohol, acetic acid andammonium hydroxide, controlling the moisture content thereof by heatingin an open autoclave, and closing the autoclave when the desiredmoisture content is reached.

11. The method of preparing an electrolyte for electrolytic cellscomprising the steps of admixing a polyhydroxy alcohol and acetic acid,bubbling ammonia gas through the admixed polyhydroxy alcohol and aceticacid,,and heating the resultant liquid in a closed autoclave whichprevents ingress and egress of any substantial amount of moisture. 1

12. The method of preparing an electrolyte for electrolytic cellscomprising the steps of admixing a polyhydroxy alcohol and ammoniumacetate, and heating the resultant liquid in a closed autoclave whichprevents ingress and egress of any substantial amount of moisture.

13. As a substantially neutral non-aqueous electrolyte of relativelyhigh fluidity for an electrolyticcell, the composition comprising there,-

prises admixing with a polyhydroxy alcohol chemical compounds includingthe acetic acid radical, and a, member of the group characterized by thecompound ammonia in quantities suflicient to preclude the presence ofany substantial quan- 5 tity of free acid in solution.

ALEXANDER M. GEORGIEV.

